A transistor structure may include a termination region on one or more outside edges of the active region of the structure. In the termination region of a power transistor, such as a metal-oxide-semiconductor (MOSFET) device, regions of high electric field develop under the application of high drain voltages. Once the electric field reaches a critical level, impact ionization of the transistor materials in the high field region can generate large quantities of carriers, leading to avalanche breakdown of the device. The drain voltage at which this avalanche process occurs is the breakdown voltage (BVdss) of the device. The breakdown voltage is an important characteristic of the device, especially when considering possible applications for the device.
The spatial location and the distribution of the electric field developed during breakdown are also important. For example, in device structures that include trenches in the termination region, a high magnitude electric field often occurs at the corners of the trenches. This electric field can promote carrier injection into the insulating regions nearby. When injected carriers are trapped in the insulating regions, they can cause trapped electric charge to accumulate in these insulating regions. This can result in drift of the device's electrical parameters (e.g., breakdown voltage, on-resistance, threshold voltage, etc.). It can also result in a reduced time to breakdown of the insulating material in this region or shorten the life of the device.